Oil and gas wells are typically drilled by installing a conductor pipe to a first depth, then drilling the well to a second depth. Usually, a string of casing is made up by coupling together sections of pipe, each being about forty feet long, and lowering the string through the conductor pipe to the second depth. Cement is then pumped down the casing, which flows back up the annulus between the casing and the open borehole. Drilling is resumed to a third depth and the process is repeated with another smaller diameter nested casing. An even smaller diameter string of casing may be installed to a fourth depth.
Casing serves to support the wellbore and to prevent undesired outflow of drilling fluid into the formation or inflow of fluid from the formation into the wellbore from strata other than the target production strata. The nested arrangement of successively smaller casing strings requires a relatively large drilled hole at the upper part of the wellbore due to the thickness of the casing and couplings and also due to the minimum clearance necessary between casing to displace cement in the annulus space.
U.S. Pat. No. 5,794,702 discloses a continuous collapsed casing system allowing to rapidly run a continuous metallic casing into a well. The casing is plastically coiled on large reels. An injector unit straightens the casing and folds it in horseshoe shape prior to lowering it into the well as it is uncoiled from the reel. When the casing reaches the target depth, it is then reopened mechanically or hydraulically to its unfolded shape substantially cylindrical. Unfortunately due to the existence of an elastic phase in the stain-stress curve of metallic material the final second shape spring back after the expander/gauge move out and exhibit marks on the wall where the horseshoe shape folds were. The direct consequence is that the misshapen shape restricts the access and degrades the external pressure performance of the reformed or deployed cylindrical pipe.
The U.S. Pat. Nos. 5,979,560 and 6,253,852 of the inventor describe downhole well equipment comprising lowering of sections of pipe which are folded longitudinally to allow their descent in the well, then re-inflated by internal press to regain their initial cylindrical shape.
WO 99/35368 (Shell International Research Maatschappij B.V.) discloses an alternative to the collapsed casing using round cylindrical pipe. Through a previously installed casing, an additional casing having a cylindrical form of a smaller diameter can be lowered and after having reached its target depth below the previously installed casing, its diameter can be increased by stretching the wall of the pipe using an expansion mandrel. In this case an expansion mandrel is pushed or drawn through the additional casing, the expansion mandrel comprising a first diameter equal to the internal diameter of the additional casing and a second diameter equal to the internal diameter that is desired to obtain, these two diameters being connected by a truncated cone. It is on this truncated cone that the plastic expansion of the wall of the pipe occurs. The expansion factor, which is the percentage of internal diameter expansion, must be at least in the order of 15%, and it is the axial length of this cone which limits the thickness of the additional casing due to the risk of initiating cracks from micro defects in the plastically expanded zone.
These various methods thus allow the installation of tubulars of a given diameter through a previously installed casing forming a reduced opening, by requiring a plastic expansion in one single step, to obtain the tubular's final dimensions in a cavity located below the previously installed casing.
But these methods result either in an irregular section of the pipe, which keeps the fold marks made during folding, in the case the section was initially folded, or in a thickness limitation, in the case where the pipe is initially cylindrical and is simply expanded. Due to these irregularities or reduced thickness, the external pressure resistance of the cylindrical pipe is largely decreased compared to the equivalent casing installed in a traditional way, without plastic deformation.